Current practice in mobile radio transceivers is to derive the operating frequencies with phase locked loop synthesizers. In such a synthesizer, a phase locked loop circuit controls a variable frequency oscillator, i.e., a voltage controlled oscillator (VCO), to lock in at the frequency and phase angle of a standard reference frequency. In this fashion, the VCO will have the same accuracy as the standard. The phase locked loop operates to track the operation of the reference oscillator.
In general, a phase locked loop includes a VCO, a reference oscillator, a phase detector and a loop filter. The phase detector compares the phase of the VCO output signal with the phase of the signal from the reference oscillator. The phase detector generates an output signal related to the difference between the two compared signals. This output signal is processed by a low pass filter and is then applied as a control signal to an input terminal of the VCO to control the frequency of the oscillator output signal.
Modulation of the VCO output signal has heretofor typically been accomplished by applying a modulation signal to either the reference oscillator, the VCO or to both the reference oscillator and the VCO. If the reference oscillator is modulated, the modulation response of the phase locked loop has a transfer function which is relatively flat for low frequencies up to approximately the natural resonant frequency of the phase locked loop. The transfer function, however, decreases rapidly at approximately the natural frequency of the phase locked loop. It is therefore generally impractical to use this method of modulation for frequencies greater than the natural frequency of the loop because of the rapidly decreasing loop response at such frequencies.
Such a method is not a satisfactory approach for frequency modulating a VCO in typical FM transmitter applications due to the lack of a high frequency response. The phase locked loop behaves as a low pass filter with respect to modulation applied to the reference oscillator. The ability of the VCO output to track the reference source frequency is limited by the effective loop gain of the feedback control loop.
Alternatively, if the VCO is modulated directly, then the loop behaves as a high pass filter with respect to applied modulation. The modulation response while relatively flat for higher frequencies decreases rapidly for low frequencies, thereby exhibiting a less than satisfactory modulation response characteristic.
In order to achieve wideband modulation, one approach that has been employed is to modulate both the VCO and the reference oscillator in a complementary fashion. In this regard, by modulating both the reference source and the VCO in the proper ratio, a flat modulation response, i.e., a constant modulation level over a wide frequency band, can be obtained.
The viability of a particular method of modulating a phase locked loop is highly application dependent. In this regard, if the phase locked loop bandwidth is large, when compared with the modulation input, then the reference oscillator should be modulated. If it is desirable to handle the audio spectrum which ranges from 200 Hz to 4 KHz, and if the phase locked loop is able to track to 1 KHz, then the loop will track up to the 1 KHz level and then be unable to satisfactorily track frequencies above 1 KHz. Under such circumstances, it may be possible to modulate the reference oscillator to achieve satisfactory operation.
On the other hand, if the phase locked loop only has a tracking bandwidth of 100 Hz, then the reference oscillator can not be modulated to achieve satisfactory system operation. Thus, based on the limitations of a particular system in a given application context, modulation of the reference oscillator, the VCO or both may be in order.
In many applications, it is undesirable or impossible to modulate the reference source. In this regard, it is often desirable to make the reference oscillator more rock solid stable with respect to its major intended purpose--namely, accurately defining a reference frequency onto which the phase locked loop can lock. This increased stability requirement is inherently inconsistent with an ability to readily modulate or change the frequency of the reference oscillator.
In other applications, the reference frequency source may be a frequency standard which is not accessible. Thus, in such a system which does not include a reference oscillator, it is impossible to modulate the reference source.
The present invention is a single input system which creates a flat frequency response down to very low frequencies and permits wideband modulation of the VCO. In contrast to many prior art attempts to provide wideband modulation of the phase locked loop, the present invention does not require separate inputs at different points in the control loop. Instead, a modulation input port to the VCO is the only modulation input that needs to be used.
In the present invention, wideband modulation of the VCO is achieved by interposing the inverse analogue of the closed phase locked loop transfer function between the modulation source and the VCO modulation input. A first branch of the interposed circuit directly passes high frequency signals from the modulation source to a summing device tied to the input of the VCO. A second branch of the interposed circuit boosts low frequency signals and blocks high frequency signals received from the modulation source and passes such signals to the summing device where signals from the first and second branches are summed to permit wideband modulation of the VCO. All the modulation processing is performed outside the phase locked loop.